The recent trend for achieving an increasingly higher level of integration of semiconductor devices has kept requiring for forming ever finer patterns. To meet such a requirement, when the patterns are formed by employing a dry etching method in a photolithography process, a resist film needs to be very thinly formed on a semiconductor wafer in order to obtain a high resolution for the fine patterns. However, as the thickness of the resist film is kept being reduced to keep up with the fine patterns, a sufficiently high selectivity of a target layer to be etched against the resist film cannot be secured, which in turn makes it difficult to form the fine patterns with a high precision.
As a way to resolve such difficulties, a triple layer resist has been utilized. An example of the triple layer resist is the one having a thick lower resist film made of an organic material, formed on a base layer to be etched for planarization; an intermediate layer made of an inorganic material such as a silicon oxide film, formed on the lower resist film by, e.g., SOG (spin on glass); and an upper resist film (photosensitive resist film) formed on the intermediate layer.
In such a triple layer resist, the upper resist film, which is the uppermost layer, is patterned first by using a photolithography technique, and then the intermediate layer is etched by using the patterned upper resist film as a mask, whereby the pattern of the upper resist film is transferred to the intermediate layer. Thereafter, an etching (dry development) of the lower resist film is carried out by using the upper resist film and the intermediate layer as a mask, so that the pattern of the upper resist film and that of the intermediate layer are transferred to the lower resist film.
Here, the upper resist film formed thinner than the lower resist film disappears in the course of the etching of the lower resist film, so that the etching of the lower resist film is completed by using the intermediate layer as a mask. After the etching, only the patterned lower resist film and intermediate layer are left, and then the base layer is etched by using laminated mask structure including the lower resist film and the intermediate layer.
When performing the dry developing process in such a series of etching processes, that is, to etch the lower resist film by using the upper resist film and the intermediate layer as a mask, an O2 based etching gas has been used (see, for example, Patent References 1 to 3).
[Patent Reference 1]
Japanese Patent Laid-open Application No. 2002-93778 (for example, claim 3)
[Patent Reference 2]
Japanese Patent Laid-open Application No. 2002-110643 (for example, claim 4)
[Patent Reference 3]
Japanese Patent Laid-open Application No. 2004-71768 (for example, claim 1)
When performing the dry developing process by using the O2 based etching gas, it is difficult to etch the lower resist film vertically, thereby resulting in a poor controllability over an etching profile. Further, when performing dry development of a pattern with a wide opening area, residues of reaction products from the etching tend to be attached to thereby be accumulated on the surface of the base layer that should to be exposed through the opening after the dry etching development. Moreover, control over the etching profile and suppression of the generation of the etching residues are in a trade-off relation in the dry developing process performed by the plasma of the O2 based etching gas, and no etching condition satisfying these two requirements has been found yet.